Most has two closely-related meanings.
1 the largest in number or amount
2 more than half / almost all of somebody/something
As that OALD link says, the most is often used for the first meaning - but informally, the is often omitted. It's never used with the second meaning.
Suppose The Queen of Hearts were to ask "Who ate my tarts?"...
A1: "The knave ate the most"
A2: "The knave ate most"
If several courtiers ate one tart each, but the knave ate two, then either answer could validly be given, since he ate more than anyone else (but note that A1 is a "very slightly formal" usage).
But if the knave actually ate more than half (but not all) of the tarts, only A2 is correct. Though the Queen might not understand that intended distinction, since the could be omitted anyway.
It's worth making the point that the is never included when most simply means very...
"I am most grateful for your help"
...but interestingly, whereas in...
"There were 50 people there, at [the] most"
... the is normally omitted, it's almost always included if we add very as an intensifier...
"There were 50 people there, at the very most"
When you say most people or most books you are speaking in general terms about the majority of people (in a country, in the world) and the majority of books (eg. are soft-back or are now available online).
Most people would agree with me .....
Most people would object to your views...
When you speak of most of the people you are talking about people in a particular context and not universally:
Most of the people in the hall....
Most of the people who went to university...
The same applies to books:
Most of the books in the library....
Most of the books that I have read...
Thus most of the documents would refer to specific documents
Most documents would refer to documents in general.
Best Answer
Using the definite article the only works if you're talking about a specific group of young people.
If you're talking about young people in general, then you need the null article.
With an singular noun, you need to use “most of the …”, e.g. “he ate most of the cake”. Grammatically, the situation looks different. But there is actually a reason behind this. In “most of the young students at this school”, most qualifies a specific domain (the young students at this school). In “most of the cake”, most qualifies a specific domain (this one cake). In “most young people”, most qualifies the non-specific group of all young people. Since there is no specific domain, there is no definite article.